Where the Sun actually stands
Signs are thirty degree slices of the ecliptic, and knowing which slice the Sun occupied on your birthday tells you far less than knowing where inside it. On 20 December the Sun has already covered most of Sagittarius. Taking noon across the years from 1930 to 2035, it falls between about twenty seven point seven and twenty nine point one degrees, so it is always in the closing stretch and never near the middle of the sign.
That narrow spread is not a coincidence and it is not luck. The tropical year runs a little under three hundred and sixty five and a quarter days, and the leap day added every fourth February pulls the calendar back into line with the orbit almost exactly. The small residue drifts slowly, which is why the position on a fixed calendar date wanders by only a fraction of a degree over decades rather than sliding through the whole sign.
The third decan and its Chaldean ruler
Each sign divides into three bands of ten degrees called decans, and this date always lands in the third band of Sagittarius, from twenty degrees to thirty. In the Chaldean scheme that band is assigned to Saturn, so the tradition reads it as Jupiter's expansive sign with a cold and structural planet laid over its final third.
Read as tradition rather than fact, that combination is usually described as the part of Sagittarius most interested in finishing what it started: the philosophy of the sign arriving at a system, the traveller writing the account. It is the layer that separates one Sagittarius birthday from another, and it is almost never mentioned on pages that stop at the sign name.
What the date does not fix
The Sun is the slowest moving thing in your chart from a birthday's point of view, which is exactly why it is the only part a calendar date can pin down. Everything faster stays open. The Moon crosses a sign in roughly two and a half days, so on any 20 December it will have been in one of two signs, and in some years the change happens mid morning or late at night, putting two people born the same day on either side of it.
The rising sign is looser still. It turns through the entire zodiac once a day, so a 20 December birth can carry any of the twelve, decided by the hour and the latitude of the birthplace rather than by the date at all. That single placement usually shapes a chart reading more than the Sun does, and no birthday page can supply it.
Using the widget rather than the average
Everything above describes the date in general. The widget above does the narrower job: give it your year, your birth time and your birthplace and it returns the exact longitude the Sun held for you, not the range for the century. It also reports the distance from the nearest sign boundary, which on this date will be small enough to be interesting without ever being in doubt.
If your time of birth is unknown the calculation falls back to noon. On a date like this one that costs you very little, since the Sun moves about a degree a day and cannot reach Capricorn from here before the day is out. It costs a great deal for the Moon and the Ascendant, and those are the placements worth chasing a birth certificate for.
Questions people ask about this
What sign is 20 December?
Sagittarius, without qualification. The Sun does not reach Capricorn until the solstice a day or two later, so no year since records began has put this date on the other side of the boundary. The only variable is the exact degree.
What degree of Sagittarius is 20 December?
Close to twenty eight. Measured at noon across the years from 1930 to 2035 it stays between about twenty seven point seven and twenty nine point one degrees. The widget above gives the figure for your own year and hour.
Which decan does this birthday fall in?
The third decan of Sagittarius, covering twenty to thirty degrees. Its Chaldean ruler is Saturn, so the tradition reads this stretch as Jupiter's sign given a more disciplined and conclusive character.
Am I on the Sagittarius Capricorn cusp?
The Sun is near the end of Sagittarius, but a cusp is not a state a chart can be in. At any instant the Sun holds one longitude and that longitude sits inside one sign. Being at twenty eight degrees makes you no less Sagittarius than being at ten.
Does the degree change anything about the reading?
It changes which decan applies and which aspects the Sun makes to other planets, both of which a traditional reading uses. It does not weaken or dilute the sign itself.
Why do other 20 December pages all read alike?
Because they answer only the sign question, which for this date has one settled answer and needs one sentence. The degree, the decan and the placements the date leaves open are where the differences between two people born on the same day actually live.